Elucidation of the host innate antiviral effector mechanisms that control HIV-1 replication at the site of entry during the earliest stages of infection could be instrumental for the development of an effective AIDS vaccine. Our laboratories have made significant progress toward this objective by demonstrating that transmitted founder (TF) viruses that have crossed the mucosa and initiated a productive systemic infection are considerably more resistant to the antiviral effects of type 1 interferons (IFNs) than viruses that predominate during chronic infection (Parrish et al., PNAS, 2013; Fenton-May et al., submitted). These findings suggest that antiviral genes up-regulated by type 1 IFNs exert significant selective pressure on the transmitted HIV-1 pool, resulting in the establishment of systemic infection by variants that are relatively IFN resistant. In this application, we propose to capitalize on this discovery by identifying the IFN-stimulated antiviral genes that counteract HIV-1 replication at the site of entry and by determining whether these newly-identified effector mechanisms can be harnessed for vaccine design. Our working hypothesis is that understanding the host effector mechanisms that control HIV-1 during the earliest stages of infection will lead to new interventions that are capable of impairing virus acquisition and initial spread. We have established a novel virus-based approach that will allow us to dissect the factors that contribute to the early anti-viral state in the mucosa by characterizing the particular interferon stimulated genes (ISGs) involved, along with the TF virus determinants that confer resistance to their activity. Specific Aims are: 1. To quantify the contribution of type 1 IFN resistance to HIV-1 transmission fitness. 2. To map the viral determinants that confer type 1 IFN resistance on transmitted founder viruses. 3. To identify the IFN-stimulated genes (ISGs) that exert antiviral pressure on HIV-1 during the earliest stages of infection. 4. To compare the ability of different vaccination regimes to induce antiviral ISGs that play an important role in early HIV-1 control. We expect these studies to provide important new insight into the capacity of different vaccination strategies to induce antiviral effector mechanisms that control HIV-1 replication at or near the site of transmission, acting in the hours and days immediately following exposure when the virus is most vulnerable.